Ensures float division for certain pmt/pv combos
Ensures float division for certain pmt/pv combos

In the case that both pmt and pv are integers, we need to insure that
we're performing floating point division, else our r_1 and r_2 guesses
are terribly inaccurate, which, when fed into our secant function, can
result in non-convergence, and a NaN result.

lmp/croovy_finance